In electrostatography an image comprising a pattern of electrostatic potential (also referred to as an electrostatic latent image) is formed on an insulative surface by any of various methods. For example, the electrostatic latent image may be formed electrophotographically (i.e., by imagewise radiation-induced discharge of a uniform potential previously formed on a surface of an electrophotographic element comprising at least a photoconductive layer and an electrically conductive substrate), or it may be formed by dielectric recording (i.e., by direct electrical formation of a pattern of electrostatic potential on a surface of a dielectric material). Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrographic developer. If desired, the latent image can be transferred to another surface before development.
One well-known type of electrographic developer comprises a dry mixture of toner particles and carrier particles. Developers of this type are commonly employed in well-known electrographic development processes such as cascade development and magnetic brush development. The particles in such developers are formulated such that the toner particles and carrier particles occupy different positions in the triboelectric continuum, so that when they contact each other during mixing to form the developer, they become triboelectrically charged, with the toner particles acquiring a charge of one polarity and the carrier particles acquiring a charge of the opposite polarity. These opposite charges attract each other such that the toner particles cling to the surfaces of the carrier particles. When the developer is brought into contact with the electrostatic latent image, the electrostatic forces of the latent image (sometimes in combination with an additional applied field) attract the toner particles, and the toner particles are pulled away from the carrier particles and become electrostatically attached imagewise to the latent image-bearing surface. The resultant toner image can then be fixed in place on the surface by application of heat or other known methods (depending upon the nature of the surface and of the toner image) or can be transferred to another surface, to which it then can be similarly fixed.
A number of requirements are implicit in such development schemes. Namely, the electrostatic attraction between the toner and carrier particles must be strong enough to keep the toner particles held to the surfaces of the carrier particles while the developer is being transported to and brought into contact with the latent image, but when that contact occurs, the electrostatic attraction between the toner particles and the latent image must be even stronger, so that the toner particles are thereby pulled away from the carrier particles and deposited in the desired amount on the latent image-bearing surface. In order to meet these requirements for proper development, the level of electrostatic charge on the toner and carrier particles should be maintained within an adequate range.
Toner particles in dry developers often contain material referred to as a charge agent or charge-control agent, which helps to establish and maintain toner charge within an acceptable range. Many types of charge-control agents have been used and are described in the published patent literature. However, the level of charge that will be created and maintained on the toner is still very dependent on the nature and condition of the carrier particles.
Many known dry, two-component electrostatographic developers contain thermoplastic toner particles and carrier particles that comprise a core material coated with a polymer. Such polymeric carrier coatings can serve a number of known purposes. One such purpose can be to aid the developer to meet the electrostatic force requirements mentioned above by shifting the carrier particles to a position in the triboelectric series different from that of the uncoated carrier core material, in order to adjust the degree of triboelectric charging of both the carrier and toner particles. Another purpose can be to reduce the frictional characteristics of the carrier particles in order to improve developer flow properties. Still another purpose can be to reduce the surface hardness of the carrier particles so that they are less likely to break apart during use and less likely to abrade surfaces (e.g., photoconductive element surfaces) that they contact during use. Yet another purpose can be to reduce the tendency of toner material or other developer additives to become undesirably permanently adhered to carrier surfaces during developer use (often referred to as scumming). A further purpose can be to alter the electrical resistance of the carrier particles.
Many different types of polymers have been described as useful for one or more of these purposes, among which are, for example, various styrene and methacrylate polymers and copolymers thereof. For example, U.S. Pat. Nos. 4,209,550; 4,572,885; and 4,822,708, and British published Patent Specification 1,385,231, suggest that, among many other polymers, poly(styrene), poly(methyl methacrylate), and poly(styrene-co-methyl methacrylate) may serve one or more of these purposes.
However, while such carrier coatings can serve the above-noted purposes well, in some cases they do not adequately serve some or all of those purposes simultaneously. For example, in some developer compositions, styrene and methacrylate polymer carrier coatings can serve many of the above-noted purposes well, but, depending upon the nature of the toner particles and carrier core material desired to be included in the developer, such carrier coatings can cause the developer to acquire a triboelectric charge that is too low for optimum developer performance. This is especially true in some negatively charged developers (developers in which the toner particles triboelectrically acquire a negative charge, and the coated carrier particles acquire a positive charge). The reason for this problem is that some of the suggested polymeric materials are not triboelectrically potent enough or efficient enough to achieve the desired degree of charging tendency of the carrier particles in certain developers.
Also, the less triboelectrically efficient or potent the polymer is for this purpose, the greater is the amount of the polymer that must be coated on a carrier core in order to achieve the desired level of charge, if that level can be achieved at all. However, two of the most desirable means of forming the coating on the core particles are solution-coating and melt-coating.
The procedure in melt-coating is to mix the core particles with finer particles of the coating material in solid form to distribute the coating particles over the core particles' surfaces, apply heat to cause the material to flow just enough to coat the core surfaces, allow the mix to cool, and then break apart the solidified mass to yield the discrete coated carrier particles. However, for example, in the case where carrier core particles comprise strontium ferrite materials and have average particle diameters in the range of about 30 to 40 micrometers, if the relative amount of polymeric coating material exceeds 3 parts per hundred parts (pph) of core material, the solidified mass becomes exceedingly difficult to properly break apart.
In solution-coating, the polymer is dissolved in appropriate solvent, the solution is mixed with carrier core particles, and the mixture is agitated while driving off the solvent to yield the coated carrier particles. Again, for example, in the case where carrier core particles comprise strontium ferrite materials and have average particle diameters in the range of about 30 to 40 micrometers, if the relative amount of polymeric material in the solution exceeds about 1.5-2 parts per hundred parts by weight of core particles, the particles can become agglomerated during the process, causing non-uniformities in the coating and limiting the amount of polymer that can be coated.
Thus, the amount of polymer that can be coated by such methods is limited (it should be noted that the specific maximum relative amounts of coating material, recited above for melt-coating and solution-coating the core particles specifically described, will be different for different core particles that may have different average particle sizes, different core material densities, and/or different surface area-to-mass ratios). The more efficient the polymer is at desirably altering the carrier particles' charging characteristics, the more desirable it is, in terms of achieving the desired charging characteristics and minimizing the amount of polymer that must be coated to achieve such characteristics.
Another drawback of some materials suggested as carrier-coating polymers is their lack of thermal stability, leading to degradation during coating and degradation during use in electrographic development, with consequent inconsistent triboelectric properties initially and over time and shorter carrier life (because of more carrier chipping, flaking, dusting, and scumming).
Thus, there remains a need for suitable polymers to be coated on carrier core particles to adjust their triboelectric charging characteristics with respect to various types of toner particles in electrographic developers. Such polymers should be highly triboelectrically potent or efficient in order to adequately modify carrier charging characteristics and minimize the amount of polymer in the coating, and should have good thermal stability. The present invention meets that need.